1. Technical Field
The present invention relates to methods for making and processing high bulk tissue webs. More particularly, the invention pertains to a method of making a tissue web that is wound on large diameter parent rolls, unwound for finishing operations, and subsequently rewound.
2. Background
Unwinds are used widely in the paper converting industry, particularly in the production of bathroom tissue and kitchen toweling. Manufactured parent rolls are unwound for finishing operations, such as calendering, embossing, printing, ply attachment, perforating, and then rewound into retail-sized logs or rolls. At the time a parent roll runs out in a traditional operation, the spent shaft or core must be removed from the machine, and a new roll moved into position by various means such as an overhead crane or extended level rails.
Historically, unwinds have made use of core plugs for support on unwind stands with the power for unwinding coming from belts on the parent roll surface. Such surface driven unwind systems are not suitable for all types of tissue webs, because they can decrease the machine direction stretch, reduce the bulk, or damage the surface of some types of tissue webs, particularly high-bulk tissue webs. In contrast, center driven unwind systems have been used mainly in film unwinding.
The down time associated with a parent roll change represents a substantial reduction in total available run time. In addition, the manpower required to change a parent roll tends to negatively impact the efficiency of a rewinder line, and possibly even the productivity of neighboring operations when workers are borrowed for roll changes. Even where a finishing unit is employed to bond the expiring web and the new web together, the webs are manually threaded and advanced resulting in significant inefficiencies. Consequently, parent roll changes according to current practices can reduce the maximum output that can be obtained from a rewinder line, and may adversely impact the productivity of neighboring operations as well.
Thus, there is a need for an improved method for making and processing a web which maintains the desirable characteristics of the web, such as the bulk and uniformity of the web. There is also a need for an improved method for making and processing a web that dramatically reduces the time the machine is actually stopped, to significantly improve overall efficiency and to maintain or improve safety for all personnel.
One aspect of the present invention pertains to a method of making and processing a high bulk tissue web. The method comprises the steps of: depositing an aqueous suspension of papermaking fibers onto an endless forming fabric to form a web; drying the web to form a dried web having a bulk of 9.0 cubic centimeters per gram or greater; winding the dried web to form a plurality of parent rolls each comprising a web wound on a core; transporting the parent rolls to an unwind stand comprising a pair of spaced apart arms, each arm comprising torque transmitting means for engaging a parent roll; engaging the torque transmitting means with a first parent roll; partially unwinding the first parent roll using variable speed drive means operably associated with the torque transmitting means; rotatably supporting the partially unwound first parent roll on a core placement table that is adapted to receive the partially unwound first parent roll from the arms; engaging the torque transmitting means with a second parent roll; bonding a leading end portion of the web on the second parent roll to a trailing end portion of the partially unwound first parent roll to form a joined web; and rewinding the joined web.
In another embodiment, a method of making and processing a high bulk, uncreped throughdried tissue web comprises the steps of: depositing an aqueous suspension of papermaking fibers onto an endless forming fabric to form a web; transferring the web to a throughdrying fabric; throughdrying the web to form an uncreped throughdried web having a bulk of 6.0 cubic centimeters per gram or greater; winding the dried web to form a plurality of parent rolls each comprising an uncreped throughdried web wound on a core; transporting the parent rolls to an unwind stand comprising a pair of spaced apart arms, each arm comprising torque transmitting means for engaging a parent roll; engaging the torque transmitting means with a first parent roll; partially unwinding the first parent roll using variable speed drive means operably associated with the torque transmitting means; rotatably supporting the partially unwound first parent roll on a core placement table that is adapted to receive the partially unwound first parent roll from the arms; engaging the torque transmitting means with a second parent roll; bonding a leading end portion of the web on the second parent roll to a trailing end portion of the partially unwound first parent roll to form a joined web; and rewinding the joined web.
The unwind stand may include a frame with pivotally mounted arms. The arms desirably move the first parent roll to an unwind position for partially unwinding the first parent roll; then move the first parent roll to a position in close proximity to or contact with the core placement table; and then move the second parent roll to an unwind position for partially unwinding the second parent roll core. When the webs from the first and second parent rolls are being spliced together, the variable speed drive means and a core placement drive motor simultaneously unwind the first and second parent rolls.
The webs of the parent rolls are desirably united using a thread-up conveyor. The leading end portion of the web on the second parent roll is transported by the thread-up conveyor, which preferably comprises a vacuum means operably associated with an endless screen belt means. In one embodiment, the leading end portion of the web on the second parent roll is transported over the endless screen belt means with decreasing amounts of vacuum. Once the leading end portion of the web on the second parent roll is disposed on the trailing end portion of the web on the partially unwound first parent roll, the thread-up conveyor and unwinding of the second parent roll are operated at a same surface speed.
Advantageously, the thread-up conveyor may be moved, and in particular pivoted, relative to the second parent roll between an active position and a standby position. In the active position, the thread-up conveyor is in close proximity to or in contact with the second parent roll, whereas in the standby position the thread-up conveyor is positioned away from the parent roll.
The core placement table is desirably moveable in a direction transverse to the path of travel of the web between an inline position and a standby position. The inline position corresponds to the web centerline to enable partially unwound parent rolls to be placed on the core placement table, whereas in the standby position the core placement table is positioned away from the unwinding operation for ease of operator access.
Suitable soft, high bulk tissues for purposes of this invention include tissue sheets as described in U.S. Pat. No. 5,607,551 issued Mar. 4, 1997 to Farrington, Jr. et al. entitled xe2x80x9cSoft Tissuexe2x80x9d, which is herein incorporated by reference. The method is particularly useful for soft, high bulk uncreped throughdried tissue sheets. Such tissues suitably have bulk values of 6.0 cubic centimeters per gram or greater (before calendering), desirably about 9 cubic centimeters per gram or greater, more specifically from about 10 to about 35 cubic centimeters per gram, and still more specifically from about 15 to about 25 cubic centimeters per gram. The method for measuring bulk is described in the Farrington, Jr. et al. patent. In addition, the soft, high bulk tissues of this invention can be characterized by a relatively low stiffness as determined by the MD Max Slope and/or the MD Stiffness Factor, the measurement of which is also described in the Farrington, Jr. et al. patent. More specifically, the MD Max Slope, expressed as kilograms per 3 inches of sample, can be about 10 or less, more specifically about 5 or less, and still more specifically from about 3 to about 6. The MD Stiffness Factor for tissue sheets of this invention, expressed as (kilograms per 3 inches)-microns0.5, can be about 150 or less, more specifically about 100 or less, and still more specifically from about 50 to about 100. Furthermore, the soft, high bulk tissues of this invention can have a machine direction stretch of about 10 percent or greater, more specifically from about 10 to about 30 percent, and still more specifically from about 15 to about 25 percent. In addition, the soft, high bulk tissue sheets of this invention suitably have a substantially uniform density since they are preferably throughdried to final dryness without any significant differential compression.
Parent roll cores used in the present method preferably have an outside diameter of at least about 14 inches, and more particularly about 20 inches. The parent rolls have a face or circumferential surface, an inner core surface, and opposite end surfaces. The outside diameters of such rolls can be at least about 60 inches, and in particular about 120 inches or greater, such as about 140 inches or greater. The widths of the parent rolls, measured between the opposite end surfaces, are generally at least about 55 inches, more particularly at least about 100 inches, such as about 105 inches or greater. Consequently, the weights of the rolls may be about 2000 lbs. or more, particularly about 3000 lbs. or more, and more particularly about 4000 lbs. or more.
In particular embodiments, a center driven unwind system is employed to eliminate or reduce the following detrimental effects on the web: 1. surface damage (scuffing, tearing, etc.); 2. wrinkling of the web; 3. de-bulking; and 4. stretch loss. All of these detrimental effects are typical of a surface driven unwind on a low-density basesheet, such as an uncreped through-air-dried basesheet. These effects negatively impact the off-line finishing processes and/or the finished product. A large factor in creating these defects is the differential effects across the circumferential surface of a parent roll due to the limited contact area with the surface driven unwind belts. Specifically the possible defects are: 1. surface damage which introduces defects or tears that affect product performance and/or process runability; 2. wrinkling which impacts processes such as calendering, embossing, printing, ply-bonding, perforating and rewinding, thereby affecting finished product appearance, performance and process runability; 3. de-bulking which results in a denser web which affects product performance and preference; and 4. stretch loss which affects product performance and/or process runability.
The center driven unwind is used to preserve web attributes, such as high bulk and stretch, during the unwinding process. The web is also treated consistently across the circumferential surface of the parent roll. Other system components, such as draw control, are used to further protect the web. As an alternative to the center driven unwind, or in combination therewith, other suitable torque transmitting means may be employed to unwind the parent rolls. For example, the torque transmitting means may comprise side clamping mechanisms such as one or more inflatable bladders that engage the opposite end surfaces of the parent rolls.
The addition of a torque transmitting means that engage the opposite end surfaces of the parent rolls provides a further means of transferring torque to the roll for unwinding. This supplemental torque transfer may be desirable for relatively high bulk sheets, because the wound in tension in the roll may be reduced in order to protect the web properties. Lower wound in tension, though, adversely impacts the ability to drive the roll from the core. In high bulk sheets, using a center-driven unwind system alone creates the potential for slippage or shifting between the individual layers of the roll as well as between the initial sheet layers and the core, particularly during periods of high acceleration or deceleration. Rapid speed changes combined with a large mass moment of inertia produces high torque requirements resulting in very large circumferential forces, especially in areas near the core. The combination of large forces and lower interlayer pressures increases the likelihood of shifting between sheet layers, which leads to problems in the unwinding sequence such as web velocity or tension variability, telescoping of the parent roll and/or severe wrinkling of the web.
In one embodiment, the supplemental torque transfer means transmits torque from the unwind shaft through the roll via the one or more inflatable bladders that are in pressure contact with the opposite end surfaces of the parent roll. The bladders can be supported by a backing plate that is operatively attached to the unwind shaft. The bladders can be deflated and thus disengaged as the parent roll is unwound to smaller diameters to eliminate disturbances with the web as it is peeled away from the roll. The bladders are suitably formed of an air or fluid impermeable material that is conformable to the end surfaces of the parent rolls, for example rubber, polyurethane, other synthetic polymers, or the like. Particularly suitable materials may have a coefficient of friction of about 0.3 or greater, and particularly about 0.5 or greater.
Hence, another aspect of the present invention concerns a torque transfer device for unwinding a tissue roll that has a circumferential surface, opposite end surfaces, an inner core surface, an outside diameter of at least about 60 inches, and a width between the opposite end surfaces of at least about 55 inches. The torque transfer device includes a frame comprising a pair of arms that are spaced apart to accommodate the width of the roll therebetween. Each arm comprises a side clamping mechanism mounted thereon and adapted to engage one of the opposite end surfaces of the tissue roll. The side clamping mechanisms comprise a backing plate operably connected to and rotatable with an unwind shaft that is connected to an electric drive means. The side clamping mechanisms also comprise an inflatable bladder mounted on the backing plate and means for inflating the bladder such that the opposite end surfaces of the roll are sandwiched between the side clamping mechanisms.
The advantages attributable to the supplemental torque transfer means compared to traditional unwind assist devices, such as surface belts and rider rolls, include: low engagement pressures may be used due to the large available contact area; the circumferential surface of the roll is not damaged; torque is transmitted directly to a significant portion of the roll versus through the core and/or the circumferential surface of the roll; and operators can observe the complete circumferential surface of the roll.
Another aspect of the invention pertains to a method for making a web with dramatically less down time needed to splice parent roll webs. The method utilizes a finishing operation that substantially continuously impacts the web in order to splice the webs together. For purposes of the present invention, finishing operations that substantially continuously impact the web include embossing, crimping, and even calendering. These finishing operations desirably impact the web over the full width of the web so that a full-width splice is produced between the webs for improved strength. The term xe2x80x9csubstantially continuously impactxe2x80x9d is used herein to refer to processes that structurally modify the surface characteristics of the web, either continuously as in calendering or substantially continuously as in embossing or crimping, and that form a joined web for rewinding purposes when two webs from different parent rolls are processed simultaneously. This is in contrast to separate bonding units that are only intermittently operated to form a splice between webs from different rolls. This is also in contrast to methods that inject bonding agents, such as glue, tape, or the like, in order to splice the webs together.
Hence, one embodiment of the invention concerns a method of splicing tissue webs without glue or tape, comprising the steps of: partially unwinding a first tissue web from a first parent roll using drive motor means; transporting the first tissue web to a finishing unit comprising rolls defining a finishing unit nip; substantially continuously impacting solely the first tissue web in the finishing unit nip while the first tissue web is unwound from the first parent roll using drive motor means; partially unwinding a second tissue web from a second parent roll; transporting the second tissue web to the finishing unit using drive motor means; maintaining the first and second tissue webs moveable relative to one another upstream of the finishing unit; simultaneously unwinding both the first and second tissue webs from the first and second parent rolls using drive motor means and passing the webs jointly through the finishing unit nip to bond the webs together; and substantially continuously impacting solely the second tissue web in the finishing unit nip while the second tissue web is unwound from the second parent roll using drive motor means.
Thus, the webs from the expiring roll and the new roll are both driven through the first process nip, and are not bonded together until the first process nip. Utilizing the first finishing operation after the unwind to splice different parent roll webs together eliminates the need for separate bonding units and eliminates the need for external bonding means such as glue, tape, or the like. The present method replaces existing manual methods such as threading each new web or tying webs together.
The tissue product of this invention can be one-ply, two-ply, three-ply or more. The individual plies can be layered or non-layered (homogeneous) and uncreped and throughdried. For purposes herein, xe2x80x9ctissue sheetxe2x80x9d is a single ply sheet suitable for facial tissue, bath tissue, towels, napkins, or the like having a density of from about 0.04 grams per cubic centimeter to about 0.3 grams per cubic centimeter and a basis weight of from about 4 to about 40 pounds per 2880 square feet. Tensile strengths in the machine direction are in the range of from about 100 to about 5,000 grams per inch of width. Tensile strengths in the cross-machine direction are in the range of from about 50 to about 2500 grams per inch of width. Cellulosic tissue sheets of paper-making fibers are preferred, although synthetic fibers can be present in significant amounts.